1. Field of the Invention
The present invention relates to a dc power-supply unit which converts ac power into dc power, and can reduce a higher harmonic component of input current so as to improve a power factor.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of a choke-input type of dc power-supply unit in the prior art disclosed in, for example, "Electric Cooperative Research Paper" Vol.46, No.2, p.78 (published by Society of Electric Cooperative Research). In FIG. 1, reference numeral 1 is an ac power supply, 2 is a reactor (inductive element) serially connected to the ac power supply 1, 3 is a rectifier circuit (rectifying means) including diode bridges (rectifying devices) 4 to 7 connected to the ac power supply 1 and the reactor 2, 8 is a smoothing circuit (smoothing means) including a capacitor 9 connected in parallel with the rectifier circuit 3, 10 is the load connected to the smoothing circuit 8, and 11 is a dc power-supply unit including the reactor 2, the rectifier circuit 3, and the smoothing circuit 8.
A description will now be given of the operation. When the source voltage shown by a mark e in FIG. 1 exceeds voltage (shown by mark vc in FIG. 1) of the capacitor 9 of the smoothing circuit 8 for half period of source voltage of the ac power supply 1, the diode bridges 4 and 7 of the rectifier circuit 3 cause charging current to flow through the reactor 2 so as to charge the capacitor 9 of the smoothing circuit 8. For a while, a difference of voltage between the source voltage e and the voltage vc of the capacitor 9 increases, and the charging current for the capacitor 9 increases. However, after a time, the difference between the source voltage e and the voltage vc of the capacitor 9 decreases, and the charging current for the capacitor 9 also decreases. When the charging current reaches zero, the diode bridges 4 and 7 of the rectifier circuit 3 are set in a nonconducting state. As in the case of the half period, the diode bridges 5 and 6 of the rectifier circuit 3 cause the same charging current to flow in the capacitor 9 of the smoothing circuit 8 through the reactor 2 for the next half period.
As a result, the source voltage e of the ac power supply 1 and current input into the dc power-supply unit 11 exhibit waveforms as shown in FIGS. 2(a), 2(b). At this time, the voltage charged in the capacitor 9 is applied to the load 10.
FIG. 3 is a circuit diagram of a conventional dc power-supply unit having an enhanced power factor, disclosed in, for example, Japanese Patent Application Laid-Open No. 2-299470. In FIG. 3, descriptions of component parts identical with those of the choke-input type of dc power-supply unit in the prior art are omitted.
In FIG. 3, reference numeral 12 is a transistor serving as switching means for short-circuiting the ac power supply 1 through the reactor 2, and 13 is a diode to prevent counter-current from the capacitor 9 of the smoothing circuit 8.
A description will now be given of the operation. In FIG. 3, when the polarity of voltage of the ac power supply 1 on the side of the reactor 2 is positive, a current path in a conducting (ON) state of the transistor 12 is established to pass through the ac power supply 1, the reactor 2, the diode bridge 4, the transistor 12, and the diode bridge 7, and return to the ac power supply 1. When the polarity of the voltage of the ac power supply 1 on the side of the reactor 2 is negative, the current path in the conducting (ON) state of the transistor 12 is established to pass through the ac power supply 1, the diode bridge 6, the transistor 12, the diode bridge 5, and the reactor 2, and return to the ac power supply 1. The conducting state is held for an appropriately short period immediately after the voltage of the ac power supply 1 passes through its zero point.
On the other hand, when the polarity of the voltage of the ac power supply 1 on the side of the reactor 2 is positive, the current path in a cutoff (OFF) state of the transistor 12 is established to pass through the ac power supply 1, the reactor 2, the diode bridge 4, the diode 13, the capacitor 9, and the diode bridge 7, and return to the ac power supply 1. When the polarity of the voltage of the ac power supply 1 on the side of the reactor 2 is negative, the current path in the cutoff (OFF) state of the transistor 12 is established to pass through the ac power supply 1, the diode bridge 6, the diode 13, the capacitor 9, the diode bridge 5, and the reactor 2, and return to the ac power supply 1.
The switching operation of the transistor 12 is repeated for each positive or negative half-wave of the voltage of the ac power-supply 1. Thus, it is possible to provide operating waveform diagrams of the source voltage, the ac input current, and the transistor 12.
That is, in FIG. 4, the ac input current I.sub.s2 (FIG. 4(c)) can serve as the input current with the reactor 2 as the load during the transistor 12 is ON, and it increases for a time period when the transistor 12 is ON. Further, accumulated energy in the reactor 2 is discharged concurrently with the OFF state of the transistor 12, resulting in a damped conducting condition. Therefore, it is possible to combine ac input current I.sub.s1 (shown in FIG. 2(b)) with the input current I.sub.s2 of the conventional choke-input type of dc power-supply unit into combined ac input current I.sub.s3 as shown in FIG. 4(d).
The ac input current I.sub.s3 is applied over an entire period including time periods T1, T2, and T3 as shown in FIG. 4. Consequently, a power factor between the ac input current I.sub.s3 and the source voltage V.sub.s (FIG. 4(a)) can be improved greater than that between the source voltage V.sub.s and the input current I.sub.s1 of the conventional choke-input type of dc power-supply unit.
FIG. 4(b) illustrates the state of the transistor 12.
In addition, FIG. 6 is a circuit diagram in which the reactor 2 of FIG. 1 is connected to the dc side of the rectifier circuit 3. Since an operation of each component part in this circuit diagram is substantially identical with that in case of FIG. 1, it is possible to provide its operating waveform diagram as shown in FIG. 5(a)-(c).
The conventional choke-input type of dc power-supply unit is constructed as set forth above. Accordingly, residual voltage serving as dc voltage is left in the capacitor 9 of the smoothing circuit 8 so that the charging current can flow only during the source voltage exceeds the residual voltage, that is, only during the source voltage is in a vicinity of its peak. As a result, there are problems, for example, in that a large higher harmonic component (in particular, the higher harmonic component of odd-numbered order such as third order, fifth order, seventh order, or eleventh order) is generated in the input current, and the power factor is reduced.
Further, the conventional dc power-supply unit having the enhanced power factor is constructed as set forth above. Therefore, it is possible to improve the power factor in the dc power-supply unit greater than that in the choke-input type of dc power-supply unit. However, there is a problem in that the dc power-supply unit is not particularly useful for the best effect of power factor improvement since the transistor 12 is conducting for a constant period exclusively (i.e., immediately after the source voltage passes through the zero point). In addition, as understood from the operating waveforms shown in FIG. 5, wherein FIG. 5(a) illustrates the source voltage V.sub.s the energy accumulated in the reactor 2 excessively increases when the transistor 12 is conducting (FIG. 5(b)) so that input current can not flow for the period T3 when magnitude of the load is varied (FIG. 5(c)). As a result, there are some problems in that the power factor is reduced, the higher harmonic component of the input current increases disadvantageously, and the energy excessively accumulated in the reactor 2 causes the voltage of the smoothing capacitor 9 to increase, resulting in destruction of devices connected to the load 10 due to the overvoltage.